Cloning Disks
Disk cloning is one of many possible measures you can take to prevent data loss. Cloning duplicates the contents of a particular volume. If the original disk is bootable, its clone can also bootable. This means that you can back up not only your documents but also the operating system. The duplicate of your disk can be used to boot your computer.
Cloning the APFS file system also affects the hidden volumes: Preboot, Recovery and VM. Beginning from macOS 10.15, system and user data are stored on separate volumes which together create a disk volume group.
External drives such as HDD or SSD are the best media for a bootable disk clone.
The process of disk cloning takes quite a lot of time. Usually it isn't convenient to clone your disk frequently. Subsequent runs of the disk cloning take less time because the program updates only the modified or removed files. Planning your backup strategy, think about a combination of disk cloning with regular backups (e.g. incremental or full) that include only user files. In such a case, backing up can be more frequent (for example, daily), and disk cloning less frequent (weekly or monthly).
If you plan to use your disk duplicate as an emergency working system, the disk for a clone should be of sufficient capacity. It should be about 10% larger than all of your files take on the original disk.
It is recommended that you run the Disk Utility application to fix possible file system problems. The tools that allow you to verify your disk are located in the First Aid tab. This application is in the Applications/Utilities folder.
The disk cloning tool erases everything on the destination volume. Make sure that there are no critical files on it.
This documentation supposes that you create a system backup to be able to boot from it on the same computer.
It is recommended that you boot from your emergency disk to make sure that a copy of your operating system works well.
There are two ways to specify a disk from which your Mac should boot. If your current operating system is running, click the Apple menu and select System Preferences… Choose a disk in the Startup Disk section. If you cannot boot from the regular disk or your computer is turned off, press and hold the Option key before turning power on. Using this method, you can also boot from the Recovery volume if you need to use the Disk Utility or other computer maintenance tool.
Disk cloning requires the APFS or Mac OS Extended (HFS+) file system. This file system has several subversions such as Encrypted or Case-sensitive. It is recommended that the destination disk has the same format as the source disk. Many disks and USB flash drives have been formatted as FAT32. You can copy files on these disks but cannot use them as a startup disk until it is properly formatted.
| Source | Destination | Bootable |
|---|---|---|
| Mac OS Extended | Mac OS Extended | Yes |
| APFS | No | |
| APFS before macOS 10.15 |
Mac OS Extended | Yes |
| APFS | Yes | |
| APFS | Mac OS Extended | No |
| APFS | Yes |
The partition scheme should be the GUID Partition Table (GPT).
The destination disk should be writable. If it has any write protection, deactivate it.
The Partitioning and Formatting your Disk section gives you an idea of how to prepare a disk for creating a bootable copy of macOS.
You can select a disk image as the destination volume. This feature enables you to store a backup of your files on a network volume or external disk that otherwise is not compatible because of formatting. Another reason why you may opt to use a disk image is that it can be encrypted. This gives your data additional protection when the backup is stored in the cloud. Note that you will not be able to boot your computer using the disk image.